1. Field of the Invention
This invention relates to tandem accelerators, wherein a high-voltage terminal is used to accelerate charged particles towards the high-voltage terminal as well as from the high-voltage terminal by means of charge exchange phenomena within the high-voltage terminal.
2. Description of the prior Art
The basic optical arrangement of a tandem acceleration system is shown in FIG. 1. (The term "optical" is used herein in the context of charged-particle optics.) The system includes a tandem accelerator and an injector. The tandem accelerator includes a high voltage terminal, a voltage generator for maintaining a high positive voltage on the high voltage terminal, and acceleration tubes. A charge-particle stripper is mounted within the high voltage terminal together with a suitable gas supply to provide gas within the stripper. The operation of a tandem accelerator is well known and is disclosed, for example, in U.S. Pat. No. 3,353,107 to Van de Graaff and elsewhere. The injector includes a suitable negative ion source and a defining apertures. Negative ions from the source are focused and are then directed into the low-energy acceleration tubes. The focusing operation is such as to produce a waist at the high voltage terminal. A beam waist is desirable at the terminal so that the stripper diameter can be as small as possible and to minimize emittance increases due to small angle scattering.
The advantages of having the injection point at fixed location have been previously discussed in U.S. Pat. No. 3,423,684 by Kenneth H. Purser. Briefly, it is pointed out that it is often desirable to define properties of an ion beam including its momentum and energy and such definition is often achieved by dispersing the ions across a fixed aperture. In addition, beam monitors, such as Faraday cups and scanners, can often be most useful when these elements are located at a fixed beam waist.